1. Field of the Invention
The invention relates to a slurry hydrocarbon synthesis process with hydrocarbon hydroisomerization in the synthesis reactor. More particularly the invention relates to a slurry Fischer-Tropsch hydrocarbon synthesis process, wherein the synthesized hydrocarbon slurry liquid is hydroisomerized in the synthesis reactor, by reacting with hydrogen in the presence of a monolithic hydroisomerization catalyst in a gas lift reactor at least partially immersed in the slurry.
2. Background of the Invention
The slurry Fischer-Tropsch hydrocarbon synthesis process is now well known and documented, both in patents and in the technical literature. This process comprises passing a synthesis gas, which comprises a mixture of H2 and CO, up into a hot reactive slurry comprising synthesized hydrocarbons which are liquid at the synthesis reaction conditions and in which is dispersed a particulate Fischer-Tropsch type of catalyst. The H2 and CO react in the presence of the catalyst and form hydrocarbons. The hydrocarbon liquid is continuously or intermittently withdrawn from the synthesis reactor and pipelined to one or more downstream upgrading operations. The upgraded products may include, for example, a syncrude, various fuels and lubricating oil fractions and wax. The downstream upgrading includes fractionation and conversion operations, typically comprising hydroisomerization, in which a portion of the molecular structure of at least some the hydrocarbon molecules is changed. It would be an improvement if the synthesized hydrocarbon slurry liquid could be at least partially hydroisomerized to reduce its pour and melt points within the synthesis reactor, to make it more transportable by pipeline before it is transferred to operations downstream and without the need for a separate hydroisomerization facility.
The invention relates to a slurry Fischer-Tropsch hydrocarbon synthesis process in which the synthesized hydrocarbon slurry liquid is hydroisomerized in the synthesis reactor by circulating it up through one or more gas lift reactors at least partially immersed in the synthesis slurry, in which the liquid reacts with hydrogen in the presence of a hydroisomerization catalyst and preferably a monolithic hydroisomerization catalyst, to hydroisomerize the liquid which is then passed back into the slurry body in the synthesis reactor. The slurry liquid, which comprises synthesized hydrocarbons that are liquid at the synthesis reaction conditions, comprises mostly normal paraffins and the hydroisomerization reduces its pour and melt points, thereby making it more pumpable and pipelinable. By gas lift reactor (hereinafter xe2x80x9clift reactorxe2x80x9d) is meant a reactor inside the synthesis reactor immersed in the slurry body therein, and wherein circulation of slurry from the surrounding slurry body, up into its interior and back out and into the surrounding slurry body, is achieved all or mostly by the lift action of hydrogen treat gas passed into it. By immersed in the slurry body in the practice of the invention, is meant wholly or mostly immersed in it. The lift reactor may comprise a simple substantially vertically oriented, hollow fluid conduit, such as a pipe open at its top and bottom and containing a hydroisomerization catalyst in its interior, along with means for injecting the hydrogen treat gas into its interior. The lift reactor(s) may be regarded as a form of lift tube or riser reactor. The process comprises contacting hot slurry from the slurry body with means for removing gas bubbles, and preferably both gas bubbles and particulate solids from it, to produce at least a gas bubble reduced slurry which, along with a hydrogen treat gas, is then passed up into the one or more gas lift reactors in which the slurry hydrocarbon liquid is at least partially hydroisomerized and then passed back into the slurry body. The hydroisomerizing catalyst located in the interior of the gas lift reactor comprises the hydroisomerization zone which is surrounded by the slurry body, but it isolated from direct contact with it. This enables hydroisomerizing the slurry liquid (i) inside the synthesis reactor and (ii) while the synthesis reactor is producing hydrocarbons, but without interfering with the hydrocarbon synthesis reaction. The concentration of hydroisomerized hydrocarbon liquid in the synthesis reactor continues to increase until equilibrium conditions are reached. When the synthesis reactor reaches equilibrium, it is possible for the slurry liquid being removed from it to comprise mostly hydroisomerized hydrocarbons of reduced pour point. In some cases, no further hydroisomerization of the liquid hydrocarbon product withdrawn from the synthesis reactor is necessary. Thus, the process of the invention will reduce and in some cases even eliminate the need for a separate, stand-alone hydroisomerization reactor and associated equipment, downstream of the synthesis reactor. If a downstream hydroisomerization reactor is needed, it will be smaller than it would be if the synthesized hydrocarbon liquid passed into it was not at least partially hydroisomerized. While all of the hydroisomerized hydrocarbon liquid is typically returned back into the surrounding slurry body in the synthesis reactor with which it mixes, in some embodiments a portion of the hydroisomerized liquid may be passed from the lift reactor, directly out of the syntheses reactor to downstream operations.
The gas bubble and preferably the slurry gas bubble and particulate solids removal means is also located in the slurry body in the synthesis reactor and may comprise the same or separate means. While various filtration means may be used to separate the slurry liquid from at least a portion of the catalyst and any other particles, before it is passed up into the hydroisomerization zone, in the practice of the invention the use of filtration means may be avoided by using known slurry solids reducing means that do not employ filtration. Simple gas bubble and solids removal means suitable for use with the present invention and which operate on density differences and gravity are known and disclosed in, for example, U.S. Pat. Nos. 5,866,621 and 5,962,537, the disclosures of which are incorporated herein by reference. Simple gas bubble removing means are disclosed in U.S. Pat. Nos. 5,382,748; 5,811,468 and 5,817,702, the disclosures of which are also incorporated herein by reference. In these patents, the gas bubble and the gas bubble and solids removal means are immersed in the slurry body and comprise the slurry entrance at the top of a downcomer, while the simple gas bubble removal means are located at the top of a downcomer and the bottom of a rejuvenation tube, which is a form of lift reactor. In the ""468 patent, a lift reactor rejuvenation tube is fed a gas bubble-reduced slurry by means of a downcomer immersed in the slurry, which turns up into the rejuvenation tube. Gas bubble removal increases the density of the slurry, so that the density of gas bubble-reduced slurry passing from the slurry body in the synthesis reactor into the bottom of the lift reactor is denser than the surrounding slurry body. This acts somewhat against the lift action of the hydrogen treat gas passed into the lift reactor. Therefore, in some cases it is preferred that the gas bubble removal take place as high up in the slurry body as possible, to provide a density-difference hydraulic driving force, in addition to the lift action of the hydrogen or hydrogen treat gas passed or injected into the hydroisomerization zone, to assist slurry circulation up through and out of the lift reactor. Such means may be located proximate or part of the entrance to a downcomer means or conduit which passes the densified, gas bubble or gas bubble and solids-reduced slurry down and into the bottom of the lift reactor. Removing gas bubbles from the slurry prior to hydroisomerization also reduces its CO and water vapor content, which could otherwise react with the hydroisomerization hydrogen and also adversely effect the hydroisomerization catalyst. A monolithic hydroisomerization catalyst having substantially vertical fluid flow channels and a minimal solid cross-sectional area perpendicular to the flow direction of the fluid minimizes the pressure drop of the fluid flowing down and across the catalyst surface. Removing catalyst and other solid particles, such as inert heat transfer particles, from the slurry upstream of the hydroisomerization zone, reduces scouring of the monolithic catalyst, plugging of the hydroisomerization reaction zone and also reduces the liquid phase viscosity.
The invention comprises a slurry Fischer-Tropsch hydrocarbon syntheses process in which synthesized hydrocarbon slurry liquid is hydroisomerized in the synthesis reactor during hydrocarbon synthesis, by circulating slurry from the slurry body in the synthesis reactor up through a hydroisomerization zone, in a lift reactor immersed in the slurry body, in which the slurry hydrocarbon liquid reacts with hydrogen in the presence of a hydroisomerization catalyst. Slurry circulation between the downcomer reactor and slurry body is achieved by contacting a portion of slurry from the slurry body with gas bubble removal means to density the slurry. At least a portion of the slurry liquid is hydroisomerized which reduces its pour point. The hydroisomerized slurry leaves the lift reactor and all or most of it passes back into the surrounding slurry body with which it mixes. Preferably the hydroisomerization catalyst comprises a monolithic catalyst and at least a portion of both solids and gas bubbles are removed from the slurry before it contacts the hydroisomerization catalyst. More specifically the invention comprises a hydrocarbon synthesis process, which includes hydroisomerizing hydrocarbon liquid produced by the synthesis reaction while the hydrocarbon liquid is being produced from a synthesis gas, the process comprising the steps of:
a) passing a synthesis gas comprising a mixture of H2 and CO into a slurry body in a slurry Fischer-Tropsch hydrocarbon synthesis reactor, in which the slurry comprises gas bubbles and a particulate hydrocarbon synthesis catalyst in a slurry hydrocarbon liquid;
(b) reacting the H2 and CO in the presence of the catalyst at reaction conditions effective to form hydrocarbons, a portion of which are liquid at the reaction conditions and comprise the slurry hydrocarbon liquid;
(c) contacting a portion of the slurry from the slurry body with means for removing gas bubbles, to form a slurry reduced in gas bubbles;
(d) passing a hydrogen treat gas and the gas bubble reduced slurry into a hydroisomerizing zone in one or more lift reactors immersed in the slurry body in the synthesis reactor, in which the hydrogen and hydrocarbon slurry liquid react in the presence of a preferably monolithic hydroisomerization catalyst to form a hydrocarbon liquid of reduced pour point, and
(e) passing all or a portion of the pour point reduced liquid back into the surrounding slurry body.